The present invention relates to an organic EL element maintaining stable emission characteristics for long periods of time.
The organic EL element comprises an organic EL layer of a thin film containing a phosphor organic compound sandwiched between a pair of electrodes, a cathode and an anode. The organic EL element is a spontaneous light-emitting element in which a hole and an electron are injected into the thin film and recombined to generate an exiton and which utilizes emission of light (fluorescence/phosphorescence) when the exiton is inactivated.
The most significant problem of the above-described organic EL element is to improve its durability and, above all, generation of non-light emitting portion called xe2x80x9cdark spotxe2x80x9d and the prevention of its growth. When the diameter of the dark spot grows in tens of micrometers(xcexcm), the non-light emitting portion can be confirmed by visual observation. A principal cause of the dark spot is the influence of water and oxygen and particularly the water, which has been known to influence the element most seriously even in a trace amount.
It is, therefore, necessary to exclude the water from the materials constructing the EL element. It is particularly important to purify organic materials used in a light-emitting part to exclude the water. The organic EL element is produced in a dry process by contriving to exclude the water in a container under vacuum at the time of forming the film or in a process such as sealing process and the like. Currently, however, the water can not be removed completely and the generation of the dark spot can not be completely prevented.
As described above, the principal problem of the organic EL element is complete removal of the water in the container to eradicate the dark spot to lengthen life. As one of the measures, it is known that the removal of the water can be drastically improved by sealing the container of the organic EL element with a water-trapping agent as additional drying means (for example, Unexamined Patent Publication (Kokai) No. 9-148066).
FIG. 4 is a side sectional view showing the structure of a conventional organic EL element using a water-trapping agent as drying means. The organic EL element 41 shown in FIG. 4 comprises an insulating and transparent glass substrate 42 on which is formed a transparent conductive film comprising ITO (Indium Tin Oxide), and forms an anode 45.
An organic EL layer 44 of a thin film of an organic compound material is formed on the anode 45. A cathode of a metal thin film of, for example Alxe2x80x94Li is formed on the organic EL layer 44, and a light-emitting part of a laminate comprising a the anode 45, the organic EL layer 44 and the cathode 46 is formed. The organic EL layer 44 is comprised of a hole injection layer, a hole transportation layer, an electron transportation layer, an electron injection layer, and the like including at least an organic light-emitting layer required for EL light-emission.
Around the periphery of the glass substrate 42 is fixed the metal cap 43 by adhesive 48 in an atmosphere of inert gas (for example dry nitrogen) from which water is removed completely or in a dry atmosphere by dry air. Thereby, the anode 45, the organic EL layer 44 and the cathode 46 on the glass substrate constructing the element are protected.
Inside the inner surface of the metal cap 43 is formed a recess 49 molded by press molding and the like. In the recess 49 is contained powder drying agent 47 such as barium oxide or calcium oxide as drying means. The recess 49 in which is contained the powder drying agent 47 is covered with a sheet-like water-permeable cover 50. The drying agent 47 in the recess 49 adsorbs the residual moisture in the container to form hydroxide, thereby removing the residual moisture in the container or moisture from outside.
However, in the case of the conventional organic EL element 41 as shown in FIG. 4 the drying agent 47 contained in the recess 49 of the metal cap 43 is powder. A problem is that when calcium oxide, used as the drying agent 47, adheres to the cathode 46, aluminum of the cathode and calcium oxide are chemically reacted with the water, resulting in corrosion of the thin film cathode of aluminum, and the like of the cathode 46. In order to solve the problem, it has been required that the recess 49 in which is contained the drying agent 47 is covered with the cover 50 to isolate the drying agent 47 from the light-emitting part so that the calcium oxide as the drying agent does not adhere the cathode.
Therefore, the metal cap 43 used has a special shape and the structure becomes complicated. Further, there is a problem in that a large space is required for confining the drying agent and the entire element becomes thick.
Further, there is a problem in that when the drying agent 47 is confined in the recess 49 of the metal cap 43, handling such that the organic EL element is not contaminated is difficult because the drying agent 47 is powder and workability is extremely bad.
Furthermore, there is a problem that the generation and growth of the dark spot at peripheral portion of the sealed portion of the organic EL element can not be sufficiently prevented, even if the dark spot is prevented by the drying means by the metal cap.
Therefore, the object of the present invention is to overcome the problems described above and the provision of an organic EL element capable of being made thinner equipped with a film-like drying means which can adsorb moisture effectively and in good workability to inhibit the generation and growth of the dark spot at the peripheral portion of the organic EL element.
According to the present invention, there is provided an organic EL element comprising: a laminate having a structure in which organic EL material layers are sandwiched between a pair of opposite electrodes; a sealed container into which is placed the laminate; and a drying member which is placed in the sealed container and prevents contamination of the organic EL material layer by moisture, characterised in that the drying member is formed of an organometallic compound.
Preferably, the drying member is formed of an organometallic compound illustrated by the formula (1) 
wherein R is one selected from the group consisting of alkyl group, alkenyl group, aryl group, cycloalkyl group, heterocyclic group and acyl group having at least one carbon atom, M is a trivalent metal atom, and n is an integer of more than one.
Preferably, the drying member is formed of an organometallic compound illustrated by the formula (2) 
wherein each of R1, R2, R3, R4 and R5 is one selected from the group consisting of alkyl group, alkenyl group, aryl group, cycloalkyl group, heterocyclic group and acyl group having at least one carbon atom, and M is a trivalent metal atom.
Preferably, the drying member is formed of an organometallic compound illustrated by the formula (3) 
wherein each of R1, R2, R3 and R4 is one selected from the group consisting of alkyl group, alkenyl group, aryl group, cycloalkyl group, heterocyclic group and acyl group having at least one carbon atom, and M is a tetravalent metal atom.
Preferably, the drying member of the organometallic compound is placed on an inner surface of the sealed container.
Preferably, the drying member is placed such that it acts as a protecting member covering the organic EL layers.
Optionally, the drying member of the organometallic compounds is used in combination with an inorganic drying agent.
Optionally, the drying member of the organometallic compounds is used in combination with an inorganic drying agent taking advantage of physical adsorption.
Optionally, the drying member of the organometallic compounds is used in combination with an inorganic drying agent taking advantage of chemical reaction.
Optionally, the drying member of the organometallic compounds is used in combination with the inorganic drying agent taking advantage of the chemical reaction and the inorganic drying agent taking advantage of the physical adsorption.
These and other objects and advantages of the present invention will be appreciated by those skilled in the art from the description given herein and the appended claims.